The hungry immigrant millions swarmed across the continent, laying waste the forests, skimming the fresh fertile soil, growing prosperous by destruction rather than by thrift and planful use. They caroused and swaggered like prodigals. They glorified mere acquisition, measuring a man's worth by the money he owned. As they filled the continent, the old world fever of imperial expansion entered their blood. They seized Cuba and the Philippines, Haiti and Santo Domingo. They set about building the greatest navy in the world. After a few faltering efforts to lead the warring nations to peace through conference and conciliation, they threw the weight of their wealth and numbers into the balance and with fire and sword imposed a victorious peace. And they were able to do this in the last analysis because of the enormous power of their coal supply, for coal in a modern industrial civilization means guns and munitions of war, transportation systems to set armies in the field, and the ability to supply them after they get there. America's coal-wrought wealth made her decisive in battle. Even so today her unique reserves of coal make her the arbiter between peace and war. Possessed of the richest coal fields in the world, she holds the destiny of the nations in her hands. For coal has grown to mean food and clothing and shelter, transportation and communication, and the economic surplus and the leisure without which science, invention, art, representative government, democratic education, and enlightened organized religion would atrophy and perish. Since coal means all these things, and since America owns the world's greatest available reserves of coal, it is obvious that the manner in which her people develop and govern their coal fields is of crucial importance, not only to themselves, but also to the rest of the world. Before the United States entered the World War, her people were hardly aware of this fact; even the momentous experience of the war has but dimly impressed its meaning upon the national mind. Our coal measures underlie an area of more than four hundred and sixty thousand square miles. They contain almost four thousand billion tons of lignite, bituminous, semibituminous, anthracite, and semianthracite coals. About two-fifths of the world's annual output is mined in the United States.